During the preparation of integrated circuit modules, certain epoxy compositions have been employed to hermetically seal the backside of the substrate on which is attached at least one integrated circuit chip. Such compositions are employed to protect and seal the electrically active portions of the module.
In order for a composition to be commercially acceptable for sealing integrated circuit modules, it must possess a number of very critical properties. For instance, the composition must be a nonconductor of electricity and must be capable of withstanding degradation due to exposure to various chemicals, such as organic solvents and due to the effect of being exposed to the flow of electricity over extended periods of time. In addition, the composition must be resistant to reversion to nonfully cured products under adverse conditions of elevated temperature and high humidity over extended periods of time to be usable for integrated circuit packages to be employed in modern day computers.
The compositions must also possess significant strength characteristics so as to resist deterioration when subjected to mechanical stresses. The compositions should also have good flexibility and be able to withstand thermal cooling (i.e. expansion and contraction due to thermal changes) without cracking. The compositions also must be resistant to permeation and diffusion of gases and solvents. However, the composition must adhere tenaciously to the particular substrate employed.
In addition, the compositions must have very carefully controlled flow characteristics so that such will flow sufficiently down the sides of the substrate to hermetically seal such to the cap or container but must not flow so far downward into the cap or can as to contact or cover the backside of the substrate. The problem of flowing into the cap or can too much is referred to as "run-in". This occurs at space gaps at the corners between the substrate which generally has a slightly rounded edge and the cap or can which is generally square. If the compositions flows completely to the underside of the substrate, then problems in reliability could be caused by destruction of solder joints upon thermal expansion. This problem has been compensated for by closely fitting the can or cap to the substrate, such as by crimping the can or cap. In addition, in view of the crucial flow characteristics needed, compositions employed today for backsealing are applied in the liquid form usually from organic solvents.